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Efficacy of entomopathogenic nematodes and fungi as biological control agents of woolly apple aphid, Eriosoma lanigerum (Hausmann) (Hemiptera: Aphididae) under South African conditions

Stokwe, Nomakholwa Faith (2016-03)

Thesis (PhD)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: The woolly apple aphid (WAA), Eriosoma lanigerum (Hausmann) is an important pest of apples, (Malus domestica Borkh.). Severe infestations by WAA can lead to colonies near spurs that can deposit honeydew on fruit, which serve as a substrate for sooty mould. If not controlled, it can lead to destruction of developing buds in the leaf axils and a reduction in tree vigour or formation of galls. Increasing interest in environmentally sustainable farming has increased the demand for environmentally friendly pest control methods. Therefore the use of naturally occurring biocontrol agents and more environmental friendly methods are needed. The study aims to determine the potential of entomopathogenic nematodes (EPNs) and entomopathogenic fungi (EPF) to control WAA.
EPNs and EPF are naturally occurring and environmentally friendly microbials that have the potential to be developed into bio-pesticides. Research on the use of EPNs and EPF as biocontrol agents against E. lanigerum showed that they have potential to be used to control WAA. This is the first report on the use of EPNs and EPF to control WAA in South Africa. These findings are significant, especially to those interested in integrated pest management (IPM).
The first objective of this study was to evaluate the virulence of endemic EPN species against WAA by conducting laboratory bioassays and to determine the effect of imidacloprid on the infectivity of EPNs. A total of seven entomopathogenic nematode isolates were evaluated for their potential as biological control agents for WAA in the laboratory, using a 24-well bioassay protocol screening method. From these, the two most virulent nematode isolates were selected. In all cases studied the degree of infection associated with mortality was less than 50% and the other tested nematode isolates were less successful. Although penetration, recovery of the non-feeding Infective Juveniles (IJ) to the feeding stage and development into the adult stage was observed in some cases, the nematodes failed to complete their life cycle inside the WAA.
When this failure to develop and reproduce was investigated using direct screening with the associated symbiotic bacteria and insect extract, it became evident that the WAA haemolymph contains an inhibitory factor that prevented the symbiotic EPN bacteria from growing. WAA size was also shown to have an effect on insect mortality, with higher mortality observed in adults and low mortality for intermediates, while no mortality was observed for the crawlers. The two most virulent nematode isolates, Nguyen, Tesfamariam, Gozel, Gaugler & Adams, 2004 and Heterorhabditis zealandica Poinar, 1990 were not affected by the addition of imidacloprid, leading to the conclusion that EPNs and imidacloprid could potentially be applied as tank mix.
The second objective of the study was to evaluate the use of two EPF, Beauveria bassiana and Metarhizium anisopliae, for the control of WAA in the laboratory. The five fungal isolates tested were pathogenic to WAA and they caused significant WAA mortality compared with the control. The commercial isolates Beauveria bassiana (Eco- Bb strain R444) and Metarhizium anisopliae (ICIPE 69) caused the highest rate of WAA mortality and were selected for further evaluation. When the effect of EPF concentration and exposure time of these two most virulent isolates were evaluated, it became evident that increased EPF concentration (1 × 107 to 1 × 1010 conidia ml-1) and exposure time increased WAA mortality. This lead to the conclusion that a relatively long incubation period and high EPF concentration are needed to achieve full efficacy of the these fungal isolates for controlling WAA.
The third objective of the study was to evaluate the interaction between EPNs and EPF for the control of WAA and the effect of mulching on EPN efficacy in pot trials. Joint use of EPNs and EPF increased insect mortality when compared to treatments with only EPNs or only EPF, indicating an antagonistic effect. Combining S. yirgalamense with B. bassiana (Eco- Bb® strain R444) and M. anisopliae (ICIPE 69) provided no advantage for improved efficacy against WAA, which lead to the conclusion that M. anisopliae (ICIPE 69) or B. bassiana (Eco- Bb® strain R444) could be used for suppression of WAA. When the environment was manipulated by adding mulches, WAA mortality was slightly higher compared to the unmulched soil. However, these differences were not statistically significant.
The fourth objective of the study was to evaluate the efficacy of S. yirgalamense, B. bassiana and M. anisopliae against WAA in the field. A single application of S. yirgalamense (80 IJs/cm2) B. bassiana (Eco- Bb® strain R444) (1g/L) and M. anisopliae (ICIPE 69) (200 ml/ha) effectively controlled WAA under South African field conditions. The EPF performed better than EPNs. Based on these results the success in controlling the root colonies resulted in greatly reduced infestations of the aerial parts of the apple trees in the trial.
These studies attempted to determine if local EPNs and EPF are pathogenic to WAA and if they could be used to control it in the field. It was found that all the isolates tested were pathogenic to WAA in the laboratory and that S. yirgalamense, B. bassiana and M. anisopliae were the most effective isolates. They were able to control WAA under field conditions, although the degree of control was low. Therefore future studies should be designed to investigate the application and post-application conditions required for EPF to be effective with respect to WAA control and to ensure that the application of the fungi is economically viable. The role of the wetting agent in improving soil saturation, penetration and uptake of EPNs and EPF also requires to be investigated.